Migration: stop recommending `--prereleases` with `pub outdated --mode=null-safety`.
Migration: stop recommending `--prereleases` with `pub outdated --mode=null-safety`.

As of
https://github.com/dart-lang/pub/commit/5fba201573fb634ab1ad023b7344f3e7c6714095,
`--mode=null-safety` implies `--prereleases`.

Change-Id: I34bc827ddd3579e968276f826c0420ffadc0eac3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/171661
Auto-Submit: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Commit-Queue: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>
Reviewed-by: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk